Microwave oven door control device

ABSTRACT

Disclosed is a microwave oven with an automatic door control device. Novel door latching/opening device which uses permanent magnets and electromagnets is provided in place of hook-type door latches and slots on a oven wall. Door state detector is also provided in which the existing permanent magnets and electromagnets rather than a separate mechanical switch are utilized to discriminate the door closed state from the door open state. Further, a processing device is employed to control the operation of the door latching/opening device and microwave energy generator in response to the input signals from the door state detector and the door open switch.

FIELD OF THE INVENTION

The present invention relates to a microwave oven; and is moreparticularly concerned with a microwave oven door control device capableof accurately detecting a faulty state of the door and facilitating thelatching and the opening operations of the door.

DESCRIPTION OF THE PRIOR ART

Generally, a microwave oven has a door to provide a user access to theinterior(cavity) of the microwave oven. The door also prevents theleakage of electromagnetic radiation during the operation of themicrowave oven.

A typical present day microwave oven employs hook-type door latches andslots on an oven wall in order to keep its door closed once the door isin the closed position. The door latches are usually secured in thelocked position during the heating operation by means of mechanicallocking structure or electrically actuated locking structure employing asolenoid. The microwave oven also has a mechanical or optical switch(orswitches) which tells a closed state from an open state. The microwaveoven can initiate and continue the heating operation only when the dooris in the closed position. Two such arrangements are disclosed in U.S.Pat. No. 3,733,456 issued to Louis W. Blackburn and Japanese PatentLaid-open Publication No. 60-33207.

A problem with such arrangements is that it is difficult and cumbersometo equip a microwave oven door with the door latches and it is alsocumbersome to form the slots on a oven wall.

Another problem with such arrangements is that the door assembly caneasily be worn away due to the physical forces used to open and closethe door.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide amicrowave oven with a novel oven door latching/opening means which usesmagnetic materials(e.g., ferromagnetics or permanent magnets) andelectromagnets in place of hook-type door latches and slots on a ovenwall.

It is also an object of the present invention to provide a microwaveoven with means for detecting the state of the door which utilizes theexisting magnetic materials and electromagnets rather than employing aseparate mechanical switch(or switches).

Another object is to provide such door latching/opening means which canbe fabricated readily and relatively economically, and which has adurable operability.

The present invention provides a microwave oven with an automatic doorcontrol device, which comprises: an enclosure having therein a heatingchamber adapted to receive an object to be heated; a door mounted at anopening of the heating chamber; at least one magnetic material providedat one side of the door; at least one electromagnet provided on theenclosure and adapted to contact with the magnetic material; at leastone door state detecting means connected with the electromagnet forgenerating a first signal based on electromotive forces, saidelectromotive forces being generated in the electromagnet when the dooris being closed or opened; a door open switch means for generating asecond signal, said second signal being generated upon pressing a dooropen switch; a processing means for generating a control signal inresponse to the first signal and the second signal; at least one doorlatching and opening means connected with the electromagnet for latchingand opening the door in response to the control signal, said door beinglatched while an electric current is applied to said electromagnet inone direction and opened when another current is applied to theelectromagnet in another direction; a microwave energy generator adaptedto radiate energy within the enclosure; and a load driving powerswitching means for operating the generator in response to the controlsignal.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present invention willbecome apparent from the following descriptions given in conjunctionwith the accompanying drawings, in which:

FIG. 1 shows a perspective view of a microwave oven embodying thepresent invention;

FIG. 2 depicts a control circuit diagram of the embodiment;

FIGS. 3A and 3B describes the respective directions of the currentsinduced in the coil of an electromagnet when a permanent magnet moveshorizontally to and from the electromagnet; and

FIGS. 4A, 4B1, 4B2, 4C1, 4C2 and 4D represent flowcharts whichillustrate the operation of the processing means shown in FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The construction of the present invention is described hereinafterreferring to the drawings, which show the preferred embodiments of thepresent invention.

In FIG. 1, the enclosure case 10 comprises a heating chamber 11 whereinan object to be heated such as food is to be placed. The heating chamber11 has a door 12 mounted by means of, e.g., hinges 13 at one e.g., left,side portion of the enclosure case 10. A control panel 14 on which adisplay unit 5 and control switches SW(e.g., a start/stop switch, a dooropen switch, etc.) may be located is provided at the other, e.g., right,side portion of the enclosure case 10. In addition, magneticmaterials(e.g., ferromagnetics or permanent magnets) 20,20' are providedat one side, for example, at the right side portion of the door 12 anddisposed so as to contact with the corresponding electromagnets 30,30'installed in the enclosure case 10.

Turning now to the sequence of heating operation with reference to FIG.1, at first, the user places an object to be heated into the heatingchamber 11 and then, closes the oven door 12. As the door is beingclosed, the permanent magnets 20,20' gets closer to the electromagnets30,30' and as a result, currents are induced in the coils of theelectromagnets 30,30'. The induced currents are used by a controlcircuit to detect the two closing stages of the door(i.e., when the dooris being closed; and when the door is completely closed).

With the door completely closed, the control circuit will apply currentsin one direction to the coils of the electromagnets 30,30' to introduceattracting forces between the permanent magnets 20,20' and theelectromagnets 30,30' until either an expected(i.e., opening by pressingthe door open switch) or an unexpected (i.e., opening by physicalforces) opening of the door happens. Accordingly, the door can bemagnetically latched once the door has been closed.

In this state, pressing the start/stop switch will initiate/end theradiation of microwaves within the heating chamber 11.

An energy generator for the generation of microwaves, under the controlof the control circuit, can radiate microwaves only while the door is inthe properly closed state. That is, when the door is not completelyclosed, the energy generator will not be initiated even if thestart/stop switch is pressed. And when either the expected or theunexpected opening happens, the energy generator once initiated willstop its microwave radiating operation.

When the user presses the door open switch to remove the heated objectfrom the heating chamber 11, currents will be applied for a period in areverse direction to the coils of the electromagnets 30,30' to introducestrong repelling forces between the permanent magnets 20,20' and theelectromagnets 30,30'. Consequently, the oven door can be opened withoutany intervening physical forces.

When the door is being opened expectedly or unexpectedly, the permanentmagnets 20,20' will gradually become distanced from the electromagnets30,30'; and, as a result, currents are induced in the coils of theelectromagnets 30,30' in the opposite direction to the previous one. Theinduced currents are used by the control circuit to detect the twoopening stages of the door (i.e., when the door is being opened; andwhen the door is completely opened).

Further, the control circuit will generate a buzzer sound with anappropriate indication on the display unit 5 when: the door is notcompletely closed, the door is being opened unexpectedly; or the door isnot completely opened upon the pressing of the door open switch.

FIG. 2 shows the control circuit diagram of an exemplary embodiment ofthe present invention.

In FIG. 2, each of the electromagnets 30,30' is connected with the doorstate detecting means 40 or 40 ' and the door latching/opening means 50or 50'. The door state detecting means 40(40') is also connected to thelines I₁ (I₃) and I₂ (I₄) of the I/O port 61 of the processing means 60.And the door latching/opening means 50(50') is also connected to thelines O₁ (O₃) and O₂ (O₄) of the I/O port 61.

The door state detecting means 40 preferably comprises a diode D₁, aresistor R₁, a condenser C₁ and an A/D converter 41 in one part and adiode D₂, resistors R₂,R₃,R₄, a condenser C₂, an amplifier OP₁, and anA/D converter 42 in the other part as shown in FIG. 2. Referring toFIGS. 2 and 3, the operations of the door state detecting means 40 willnow be fully described. In the first place, we will assume the oven dooris being closed. As the door is being closed, the permanent magnet 20with the pole directions shown in FIGS. 2 and 3 approaches theelectromagnet 30; and, as a result, electric current is induced in thecoil of the electromagnet 30 in the direction shown in FIG. 3A. Theinduced current will flow from the coil to the A/D converter 41 throughthe diode D₁, the resistor R₁ and the condenser C₁. Hence, the voltageacross the condenser C₁ will be applied to the A/D converter 41 as aninput voltage thereof. Further, as the permanent magnet 20 approachesthe electromagnet 30, the induced emf(electromotive force) willgradually be increased and so will the voltage across the condenser C₁.Accordingly, the voltage has its peak value at the point of time justbefore the permanent magnet 20 contacts with the electromagnet 30. TheA/D converter 41 then converts the time-varying analog input voltagesignal to the appropriate bits(for example, 8 bits) of a digital signal.The resulting bits of digital signal will be sent in parallel throughthe line I₁ to the processing means 60 which will be described later.

Secondly, we will assume the door is being opened expectedly orunexpectedly. As the door is being opened, the permanent magnet 20becomes more distant from the electromagnet 30; and, as a result,electric current is induced in the coil of the electromagnet 30 in thedirection shown in FIG. 3B. The induced negative current is appliedthrough the diode D₂ and the resistor R₂ to the input terminal of theinverting voltage amplifier comprising the amplifier OP₁ and theresistor R₃. Hence, the inverted positive voltage across the condenserC₂ will be applied to the A/D converter 42 as an input voltage thereof.Meanwhile, as the permanent magnet 20 moves away from the electromagnet30, the absolute value of the induced emf will gradually be decreaseduntil zero and so will the voltage across the condenser C₂. Accordingly,the voltage has its peak value at the point of time when the permanentmagnet 20 has just departed from the electromagnet 30. The A/D converter42 then converts the time-varying analog input voltage signal to theappropriate bits of a digital signal. The resulting bits of digitalsignal will also be sent in parallel through the line I₂ to theprocessing means 60.

In FIG. 2, the door latching/opening means 50 preferably comprises twotransistors TR₁, TR₂. The base of the TR₁ is connected with the line O₁of the I/O port 61 while its emitter is connected with the coil of theelectromagnet 30. Accordingly, when an output signal at the line O₁ ishigh, the transistor TR₁ will be turned on. As a result, electriccurrent will flow from a voltage source V to the ground through the coilof the electromagnet 30, which will introduce attracting forces betweenthe permanent magnet 20 and the electromagnet 30. Therefore, the ovendoor can be magnetically latched by the output signal from theprocessing means 60 once the door has been closed.

On the other hand, the base of the TR₂ is connected with the line O₂ ofthe I/O port 61 while its collector is connected with the coil of theelectromagnet 30. Accordingly, when the output signal at the line O₂ ishigh, the transistor TR₂ will be turned on. As a result, electriccurrent will flow from the ground to a voltage source -V through thecoil of the electromagnet 30, which will introduce repelling forcesbetween the permanent magnet 20 and the electromagnet 30. Therefore, theoven door can be opened by the output signal from the processing means60 upon the pressing of the door open switch.

A load driving power switching means 70 is shown on the right side tothe I/O port 61. The switching means 70 comprises two resistors, a relayand a transistor whose base is connected to the line O₅ of the I/O port61. Accordingly, as far as the output signal at the line O₅ is heldhigh, the transistor becomes turned on and the relay makes its contact.Thereby, the switching means 70 controllably connects an AC source(notshown) to the loads(e.g., microwave generator, fan, motor, etc. also notshown) of the microwave oven. That is to say, in the event of a faultysituation i.e., when the oven door is not completely closed or when thedoor is opened expectedly or unexpectedly, the output signal at the lineO₅ becomes low, which will disconnect the AC source from the loads.

A door open switch means 80 is also shown in FIG. 2 as being connectedto the line I₅ of the I/O port 61. The door open switch means comprisesa voltage divider having two resistors and the door open switch. Uponthe appropriate pressing of the switch, a low signal will be appliedthrough the line I₅ to the processing means 60.

A display means 90 and an alarm means 95 are also connected to the I/Oport 61. The display means is shown as being connected to the line O₆.It comprises mainly seven-segment LED units. The alarm means isconnected to the line O₇. It comprises two resistors, a transistor and abuzzer. These means will be activated upon the occurrence of any of thefollowing faulty situations: when the door is not completely closed, thedoor is not completely opened upon the pressing of the door open switchor the door is being opened unexpectedly.

Further, the processing means 60 shown in FIG. 2 preferably comprisesthe I/O port 61, a microprocessor 62 with a reset terminal 66, a ROM 63,a RAM 64 and a system bus 65. Although not specifically shown, thesystem bus may include a data bus, an address bus and a control bus. Theinput and the output signals of the I/O port 61, which are mentionedhereinabove, are processed or generated by the processing means 60.

The microprocessor 62 acting as a main processing unit executes acontrol program stored in the ROM 63. During the program execution,relevant data will be stored in the RAM 64. Referring to FIGS. 4A, 4B1,4B2, 4C1, 4C2 and 4D, an exemplary operation of the microprocessor 62will now be described in accordance with the specific decision logic ofthe present invention. Other decision logic may also be advantageouslyand equally adopted and stored within the ROM 63. Further, some parts ofthe following sequences may be changed or eliminated for efficiency andfor application in specific purposes.

The main sequence, shown in FIG. 4A, is repeated as long as themicrowave oven power is on. The sequence will be started from Block 101after the power on or the resetting of the microprocessor 62. After aninitialization, Block 102 reads the two input signals currently held atthe lines I₁ and I₃ of the I/O port 61(shown in FIG. 2) into ti₁ and ti₃stored in the RAM 64. When each of ti₁ and ti₃ exceeds a predeterminedvalue V₂₀₀, the sequence will be diverted to the door close processingsequence shown in FIGS. 4B1 and 4B2, which will be explained later. Thepredetermined value V₂₀₀ stored in the ROM 63 corresponds to the leastsignificant value from the lines I₁ and I₃, which would indicate thedoor being closed. The value V₂₀₀ is determined empirically.

Block 104 then reads the input signal held at the line I₅ of the I/Oport 61 into ti₅ stored in the RAM 64. When the ti₅ is low toacknowledge that the door open switch has been pressed, the sequencewill be diverted to the expected door open processing sequence shown inFIGS. 4C1 and 4C2, which will be explained later.

Block 106 then reads the two input signals currently held at the linesI₂ and I₄ of the I/O port 61 into ti₂ and ti₄ stored in the RAM 64. Wheneach of ti₂ and ti₄ exceeds another predetermined value V₃₀₀, thesequence will be diverted to the unexpected door open processingsequence shown in FIG. 4D, which will be explained later. Thepredetermined value V₃₀₀ stored in the ROM 63 corresponds to the leastsignificant value from the lines I₂ and I₄, which would indicate thedoor being opened. The value V₃₀₀ is determined through experiments. Themain sequence from the block 102 will be repeated until the power off orthe resetting of the microprocessor 62 happens.

The door close processing sequence, shown in FIG. 4B, is executed whenthe door is being closed as stated above. Block 201 sets variablesstored in the RAM 64. That is, Block 201 assigns the ti₁ and the ti₃ toti₁₋₋ max and ti₃₋₋ max respectively and assings current time from readtime clock(not shown) to ti₁₋₋ max₋₋ t and ti₃₋₋ max₋₋ t. Block 202 thenreads again the input signal currently held at the line I₁ of the I/Oport 61 into the ti₁. If the ti₁ is greater than or equal to the ti₁₋₋max previously stored, then the ti₁₋₋ max will be newly set to the ti₁and current time will be written into the ti₁₋₋ max₋₋ t at Block 204.Accordingly, the ti₁₋₋ max will be always set to the maximum inputsignal from the line I₁ upto that point. Block 205 reads the inputsignal currently held at the line I₃ of the I/O port 61 into the ti₃. Ifthe ti₃ is greater than or equal to the ti₃₋₋ max, then the ti₃₋₋ maxwill be newly set to the ti₃ and current time will be written into theti₃₋₋ max₋₋ t at Block 207. Thereafter, the subsequence from Block 202to Block 208 will be repeated until both the ti₁ and the ti₃ drop tozero.

Block 209 determines whether all of the following three conditions aremet: both the ti₁₋₋ max and the ti₃₋₋ max are greater than anotherpredetermined value V₂₅₀, the ti₁₋₋ max is equal to the ti₃₋₋ max withinthe predetermined permissable boundary E₂₀₀ ; and the ti₁₋₋ max₋₋ t isequal to the ti₃₋₋ max₋₋ t within another predetermined permissibleboundary E₂₅₀. The predetermined value V₂₅₀ stored in the ROM 63corresponds to the least significant value from the lines I₁ and I₃,which would indicate the door is completely closed. The predeterminedvalue E₂₀₀ also stored in the ROM 63 is provided for compensating theerrors which may come from the time interval between the readings of theline I₁ and the line I₃, the differences in the characteristics of theA/D converters (shown in FIG. 2), etc. The predetermined value E₂₅₀stored in the ROM 63 stands for similar purposes. The predeterminedvalues V₂₅₀, E₂₀₀, E₂₅₀ should be set through experiments. If all of thethree conditions are met, it can be recognized that the door iscompletely closed. Hence, Block 210 provides a high signal for each ofthe lines O₁ and O₃ of the I/O port 61 to magnetically latch the dooruntil either an expected or an unexpected opening happens. Block 210also provides another high signal at the line O₅ of the I/O port 61 tomake it possible to connect the AC power to the loads of the microwaveoven. In this state, pressing the start/stop switch will initiate/endthe radiation of electromagnetic energy.

On the other hand, if any of the three conditions is not met, it can berecognized that the door is not completely closed because, for example,some alien substance has intervened between the permanent magnets 20,20'(shown in FIG. 2) and the electromagnets 30,30' (also shown in FIG. 2).Therefore, Block 211 generates an appropriate display signal at the lineO₆ of the I/O port 61 to tell the user that the door is not completelyclosed and provides a high signal for the line O₇ of the I/O port 61 tomake a buzzer sound. The display means 90 and the alarm means 95 (shownin FIG. 2) will be activated until the user fixes the fault and resetsthe microprocessor 62 by using the reset terminal 66 (shown in FIG. 2).Finally, at Block 212, return to the main sequence is performed.

The expected door open processing sequence, shown in FIGS. 4C1 and 4C2,is executed when the door open switch has been pressed as stated above.

At first, Block 301 assigns current time to a variable t₀ stored in theRAM 64. Then, at Block 302, the high signal which was provided for eachof the lines O₁ and O₃ to magnetically latch the door becomes low.Further, the high signal which was provided at the line O₅ of the I/Oport 61 falls low to cut out the AC power from the loads of themicrowave oven. Block 303 then provides a high signal for each of thelines O₂ and O₄ of the I/O port 61 to introduce repelling forces betweenthe permanent magnets 20,20' and the electromagnets 30,30'. Block 304then reads the two input signals currently held at the lines I₂ and I₄of the I/O port 61 into the ti₂ and the ti₄. When each of the ti₂ andthe ti₄ exceeds the predetermined value V₃₀₀, Block 308 will be entered.The predetermined value V₃₀₀ together with the meaning thereof hasalready been mentioned in the context of the main sequence.

If the condition is not met, the time limit must be checked at Blocks306 and 307. That is, Block 306 assigns current time to a variable t₁stored in the RAM 64 and then, Block 307 determines whether (t₁ -t₀) isabove a time limit T₁ stored in the ROM 63; in other words, whether thedoor does not begin to open until the predetermined time limit T₁elapsed. If the condition is not met, the subsequence from Block 303will be repeated. If the condition is met, Block 313 will be entered,which will be described hereinafter.

At Block 308, the high signal which was provided for each of the linesO₂ and O₄ to introduce repelling forces between the permanent magnets20,20' and the electromagnets 30,30' falls low. Block 309 then againreads the two input signals currently held at the lines I₂ and I₄ of theI/O port 61 into the ti₂ and the ti₄. Thereafter, Block 310 assignscurrent time to a variable t₂ stored in the RAM 64. Block 311 thendetermines whether both the ti₂ and the ti₄ are zero. If the conditiondoes not hold, the subsequence from the block 309 will be repeated.

On the other hand, if the condition does hold, Block 312 will beentered. Block 312 determines if (t₂ -t₀) (which corresponds to theperiod that the door is being opened) is above another predeterminedtime limit T₂ stored in the ROM 63. The time limit T₂ must be longenough that it can be recognized that the door has been successfullyopened without interruption. If the (t₂ -t₀) is above the time limit,recognizing that the door is opened successfully, the sequence will bereturned to the main sequence at Block 314.

However, if it is not true, assuming that some interruption has beenmade during the period, the sequence will execute Block 313. Block 313will generate an appropriate display signal at the line O₆ of the I/Oport 61 to tell the user that the door is not completely opened, andprovide a high signal at the line O₇ of the I/O port 61 to make a buzzersound. The display means 90 and the alarm means 95 will be activateduntil the user fixes the fault and resets the microprocessor 62.

FIG. 4D shows the unexpected door open processing sequence. The sequencewill be entered when the door is being opened unexpectedly as statedabove.

At Block 401, the high signal which was provided for each of the linesO₁ and O₃ of the I/O port 61 to magnetically latch the door falls low.Further, the high signal which was provided at the line O₅ of the I/Oport 61 falls low to cut out the AC power from the loads of themicrowave oven. Block 402 generates an appropriate display signal at theline O₆ of the I/O port 61 to tell the user that the door is beingopened unexpectedly, and provides a high signal at the line O₇ of theI/O port 61 to make a buzzer sound. The display means 90 and the alarmmeans 95 will be activated until the user fixes the fault and resets themicroprocessor 62.

The above-described arrangements are, of course, merely illustrative ofthe application of the principles of the invention. Other arrangementsmay be devised by those skilled in the art without departing from thespirit or scope of the invention. For example, it will be appreciatedthat it may be advantageous in certain application to employ more thantwo pairs of magnetic material and electromagnet with a correspondingnumber of door state detecting means and door latching/opening means.Also, the processing means can advantageously be replaced by ahard-wired logic circuit.

Again, it is to be understood that the invention is not to be limited tothe disclosed embodiments; but, on the contrary, it is intended to covervarious modifications and equivalent arrangements included within thespirit and scope of the appended claims.

What is claimed is:
 1. A microwave oven comprising:an enclosure havingtherein a heating chamber adapted to receive an object to be heated; adoor mounted at an opening of the heating chamber; at least two magneticmaterials provided at one side of the door; at least two electromagnetsprovided on the enclosure and adapted to contact with the respectivemagnetic materials; at least two door state detecting means eachconnected with one of the electromagnets for generating a first signalbased on electromotive forces, said electromotive forces being generatedin the electromagnets when the door is being closed or opened; a dooropen switch means for generating a second signal, said second signalbeing generated upon pressing a door open switch; a processing means forgenerating in response to the first signal and the second signal a firstcontrol signal when the door is closed and a second control signal whenthe door is to be opened; at least two door latching and opening meanseach connected with one of the electromagnets for latching and openingthe door in response to the first and the second control signals, saiddoor being latched by applying an electric current to the electromagnetsin one direction and opened by applying another electric current to theelectromagnets in another direction; a microwave energy generatoradapted to radiate energy within the enclosure; and a load driving powerswitching means for activating the generator in response to the firstcontrol signal.
 2. The microwave oven as set forth in claim 1 furthercomprising:a second processing means for generating a display signal inresponse to the first signal and the second signal; and a display meansfor displaying a faulty symbol in response to the display signal, saidsymbol being displayed when a corresponding faulty state of the dooroccurs.
 3. The microwave oven as set forth in claim 1 furthercomprising:a second processing means for generating a second controlsignal in response to the first signal and the second signal; and analarm means for generating an alarm sound in response to the secondcontrol signal, said alarm sound being generated when a fauty state ofthe door occurs.
 4. The microwave oven as set forth in claim 1 whereinsaid processing means comprises at least one input/output port, onemicroprocessor and one memory.
 5. The microwave oven as set forth inclaim 1 wherein said magnetic material is a permanent magnet or aferromagnetic.
 6. The microwave oven as set forth in claim 1 whereinsaid processing means further comprises means for detecting the state ofthe door when the respective first signals reach a same maximum valueexceeding a predetermined value at the same time.
 7. A microwave ovencomprising:an enclosure having therein a heating chamber adapted toreceive an object to be heated; a door mounted at an opening of theheating chamber; at least two magnetic materials provided at one side ofthe door; at least two electromagnets provided on the enclosure andadapted to contact with the respective magnetic materials; at least twodoor state detecting means each connected with one of the electromagnetsfor generating a signal based on electromotive forces, saidelectromotive forces being generated in the electromagnets when the dooris being closed or opened; a processing means for generating a controlsignal in response to the signal when the door is closed; a microwaveenergy generator adapted to radiate energy within the enclosure; and aload driving power switching means for activating the generator inresponse to the control signal.
 8. The microwave oven as set forth inclaim 7 further comprising:a second processing means for generating adisplay signal in response to the signal; and a display means fordisplaying a faulty symbol in response to the display signal, saidsymbol being displayed when a corresponding faulty state of the dooroccurs.
 9. The microwave oven as set forth in claim 7 furthercomprising:a second processing means for generating a second controlsignal in response to the signal; and an alarm means for generating analarm sound in response to the second control signal, said alarm soundbeing generated when a fauty state of the door occurs.
 10. The microwaveoven as set forth in claim 7 wherein said processing means comprises atleast one input/output port, one microprocessor and one memory.
 11. Themicrowave oven as set forth in claim 7 wherein said magnetic material isa permanent magnet or a ferromagnetic.
 12. The microwave oven as setforth in claim 7 wherein said processing means further comprises meansfor detecting the state of the door when the respective signals reach asame maximum value exceeding a predetermined value at the same time.